88 research outputs found
Linear Operation of Switch-Mode Outphasing Power Amplifiers
Radio transceivers are playing an increasingly important role in modern society. The
”connected” lifestyle has been enabled by modern wireless communications. The demand
that has been placed on current wireless and cellular infrastructure requires increased spectral
efficiency however this has come at the cost of power efficiency. This work investigates
methods of improving wireless transceiver efficiency by enabling more efficient power
amplifier architectures, specifically examining the role of switch-mode power amplifiers in
macro cell scenarios. Our research focuses on the mechanisms within outphasing power
amplifiers which prevent linear amplification. From the analysis it was clear that high power
non-linear effects are correctable with currently available techniques however non-linear effects
around the zero crossing point are not. As a result signal processing techniques for suppressing
and avoiding non-linear operation in low power regions are explored. A novel method of digital
pre-distortion is presented, and conventional techniques for linearisation are adapted for the
particular needs of the outphasing power amplifier. More unconventional signal processing
techniques are presented to aid linearisation of the outphasing power amplifier, both zero
crossing and bandwidth expansion reduction methods are designed to avoid operation in nonlinear
regions of the amplifiers. In combination with digital pre-distortion the techniques
will improve linearisation efforts on outphasing systems with dynamic range and bandwidth
constraints respectively.
Our collaboration with NXP provided access to a digital outphasing power amplifier,
enabling empirical analysis of non-linear behaviour and comparative analysis of behavioural
modelling and linearisation efforts. The collaboration resulted in a bench mark for linear
wideband operation of a digital outphasing power amplifier. The complimentary linearisation
techniques, bandwidth expansion reduction and zero crossing reduction have been evaluated in
both simulated and practical outphasing test benches. Initial results are promising and indicate
that the benefits they provide are not limited to the outphasing amplifier architecture alone.
Overall this thesis presents innovative analysis of the distortion mechanisms of the
outphasing power amplifier, highlighting the sensitivity of the system to environmental effects.
Practical and novel linearisation techniques are presented, with a focus on enabling wide band
operation for modern communications standards
Multi-band OFDM UWB receiver with narrowband interference suppression
A multi band orthogonal frequency division multiplexing (MB-OFDM) compatible
ultra wideband (UWB) receiver with narrowband interference (NBI) suppression
capability is presented. The average transmit power of UWB system is limited to
-41.3 dBm/MHz in order to not interfere existing narrowband systems. Moreover, it
must operate even in the presence of unintentional radiation of FCC Class-B compatible
devices. If this unintentional radiation resides in the UWB band, it can jam the
communication. Since removing the interference in digital domain requires higher dynamic
range of analog front-end than removing it in analog domain, a programmable
analog notch filter is used to relax the receiver requirements in the presence of NBI.
The baseband filter is placed before the variable gain amplifier (VGA) in order to reduce
the signal swing at the VGA input. The frequency hopping period of MB-OFDM
puts a lower limit on the settling time of the filter, which is inverse proportional to
notch bandwidth. However, notch bandwidth should be low enough not to attenuate
the adjacent OFDM tones. Since these requirements are contradictory, optimization
is needed to maximize overall performance. Two different NBI suppression schemes
are tested. In the first scheme, the notch filter is operating for all sub-bands. In the
second scheme, the notch filter is turned on during the sub-band affected by NBI.
Simulation results indicate that the UWB system with the first and the second suppression
schemes can handle up to 6 dB and 14 dB more NBI power, respectively. The results of this work are not limited to MB-OFDM UWB system, and can be
applied to other frequency hopping systems
Analog dithering techniques for highly linear and efficient transmitters
The current thesis is about investigation of new methods and techniques to be able to utilize the switched mode amplifiers, for linear and efficient applications. Switched mode amplifiers benefit from low overlap between the current and voltage wave forms in their output terminals, but they seriously suffer from nonlinearity. This makes it impossible to use them to amplify non-constant envelope message signals, where very high linearity is expected. In order to do that, dithering techniques are studied and a full linearity analysis approach is developed, by which the linearity performance of the dithered amplifier can be analyzed, based on the dithering level and frequency. The approach was based on orthogonalization of the equivalent nonlinearity and is capable of prediction of both co-channel and adjacent channel nonlinearity metrics, for a Gaussian complex or real input random signal. Behavioral switched mode amplifier models are studied and new models are developed, which can be utilized to predict the nonlinear performance of the dithered power amplifier, including the nonlinear capacitors effects. For HFD application, self-oscillating and asynchronous sigma delta techniques are currently used, as pulse with modulators (PWM), to encode a generic RF message signal, on the duty cycle of an output pulse train. The proposed models and analysis techniques were applied to this architecture in the first phase, and the method was validated with measurement on a prototype sample, realized in 65 nm TSMC CMOS technology. Afterwards, based on the same dithering phenomenon, a new linearization technique was proposed, which linearizes the switched mode class D amplifier, and at the same time can reduce the reactive power loss of the amplifier. This method is based on the dithering of the switched mode amplifier with frequencies lower than the band-pass message signal and is called low frequency dithering (LFD). To test this new technique, two test circuits were realized and the idea was applied to them. Both of the circuits were of the hard nonlinear type (class D) and are integrated CMOS and discrete LDMOS technologies respectively. The idea was successfully tested on both test circuits and all of the linearity metric predictions for a digitally modulated RF signal and a random signal were compared to the measurements. Moreover a search method to find the optimum dither frequency was proposed and validated. Finally, inspired by averaging interpretation of the dithering phenomenon, three new topologies were proposed, which are namely DLM, RF-ADC and area modulation power combining, which are all nonlinear systems linearized with dithering techniques. A new averaging method was developed and used for analysis of a Gilbert cell mixer topology, which resulted in a closed form relationship for the conversion gain, for long channel devices
Multi-band OFDM UWB receiver with narrowband interference suppression
A multi band orthogonal frequency division multiplexing (MB-OFDM) compatible
ultra wideband (UWB) receiver with narrowband interference (NBI) suppression
capability is presented. The average transmit power of UWB system is limited to
-41.3 dBm/MHz in order to not interfere existing narrowband systems. Moreover, it
must operate even in the presence of unintentional radiation of FCC Class-B compatible
devices. If this unintentional radiation resides in the UWB band, it can jam the
communication. Since removing the interference in digital domain requires higher dynamic
range of analog front-end than removing it in analog domain, a programmable
analog notch filter is used to relax the receiver requirements in the presence of NBI.
The baseband filter is placed before the variable gain amplifier (VGA) in order to reduce
the signal swing at the VGA input. The frequency hopping period of MB-OFDM
puts a lower limit on the settling time of the filter, which is inverse proportional to
notch bandwidth. However, notch bandwidth should be low enough not to attenuate
the adjacent OFDM tones. Since these requirements are contradictory, optimization
is needed to maximize overall performance. Two different NBI suppression schemes
are tested. In the first scheme, the notch filter is operating for all sub-bands. In the
second scheme, the notch filter is turned on during the sub-band affected by NBI.
Simulation results indicate that the UWB system with the first and the second suppression
schemes can handle up to 6 dB and 14 dB more NBI power, respectively. The results of this work are not limited to MB-OFDM UWB system, and can be
applied to other frequency hopping systems
A review of gallium nitride LEDs for multi-gigabit-per-second visible light data communications
The field of visible light communications (VLC) has gained significant interest over the last decade, in both fibre and free-space embodiments. In fibre systems, the availability of low cost plastic optical fibre (POF) that is compatible with visible data communications has been a key enabler. In free-space applications, the availability of hundreds of THz of the unregulated spectrum makes VLC attractive for wireless communications. This paper provides an overview of the recent developments in VLC systems based on gallium nitride (GaN) light-emitting diodes (LEDs), covering aspects from sources to systems. The state-of-the-art technology enabling bandwidth of GaN LEDs in the range of >400 MHz is explored. Furthermore, advances in key technologies, including advanced modulation, equalisation, and multiplexing that have enabled free-space VLC data rates beyond 10 Gb/s are also outlined
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